Thermodynamic engines utilize a working fluid such as air undergoing continuous or cyclic thermodynamic process in order to create mechanical work output. In most engines the working fluid is compressed then heat energy is added to it before it is expanded to produce a net work output. Increasing the amount of compression or the amount of heat energy added generally serves to increase the efficiency. In general the most desirable combination of characteristics for such a thermodynamic engine is compact size, low mass, low cost and high efficiency. Additionally the ability to operate on a variety of fuels may be advantageous for some applications. Examples of typical thermodynamic engines are given below.
Internal Combustion Engines
Amongst the most common engines are internal combustion (IC) engines in which the working fluid air, or an air fuel mixture, is compressed in an enclosed volume by the movement of a piston and is then heated by igniting the air fuel mixture (spark ignition) or by introducing and burning fuel (diesel), before being expanded against the piston to extract work. While the theoretical efficiency of a lossless internal combustion engine may be high, in practice there are many loss mechanisms that reduce their efficiency including: unwanted heat transfer from the gases to the engine walls, friction between moving parts, flow losses in valves and passages caused by high flow velocities and turbulence.
Diesel engines are generally the most efficient internal combustion engines and are up to about 40% efficient in cars, 45% in trucks, and just over 50% in very large ship engines. However, diesels also suffer from a number of serious disadvantages: they typically require vibration and noise suppression systems, they create a lot of waste heat to be dissipated by heavy and bulky coolers, they may produce unacceptable emissions requiring emissions treatment systems, they are also typically bulky and heavy making them unsuitable for many applications. It is therefore an objective of the current invention to provide an engine that is efficient and relatively quiet and vibration free when compared to IC engines, as well as requiring no or relatively little exhaust emissions treatment compared to internal combustion engines.
Internal combustion engines are frequently developed to make use of a specific fuel formulation such as Diesel, Ethanol or Natural Gas, and in some cases may not be able to run on other fuels even when those other fuels may be preferable for reasons of cost, availability or reduced emissions. It is therefore an objective of the current invention to provide an engine that has the potential to allow for running on a variety of fuels with minimal modification, or to at least provide the public with a useful choice.
Gas Turbines
Simple cycle gas turbines (GT) engines are based on the ideal thermodynamic Brayton cycle in which the gas undergoes adiabatic compression followed by isobaric heat addition, adiabatic expansion and finally isobaric heat rejection. In practice GT engines generally use a turbo compressor to compress ambient air and deliver it under pressure to a combustion chamber where fuel is burnt at approximately constant pressure producing combustion gases that are then expanded through a turbine where work is extracted to drive the compressor and supply external loads.
GT engine efficiency generally increases with increasing compressor pressure ratio, but material temperature limits and compressor and turbine inefficiencies limit the practical maximum pressure ratios and efficiencies. In practice GT engines can achieve efficiencies of about 40% in large sizes (higher than 10 MW), but lower compressor and turbine efficiencies in small sizes typically greatly reduce the efficiencies possible at power levels in the sub 100 kW range unless the GT engine is combined with heavy and bulky high temperature heat exchangers such as in recuperated or regenerated Brayton cycles.
GT engines are widely employed in aircraft, ships and power stations where they have some major advantages over IC engines including: high power to mass ratios that can exceed 10 kW/kg with little or no external cooling required. But GT engines also have a number of disadvantages when compared to IC engines of similar power: They are generally less efficient and far more expensive with high precision parts made from expensive materials. Their rotors also have large angular momentum that typically makes them slow to start up and relatively slow to respond to changes in demand. GT engines also generally have relatively limited ratios of efficient maximum to minimum power output.
These disadvantages have severely restricted the applications in which GT engines see common use. It is therefore an objective of the current invention to offer an engine that shares many of the advantages of GT engines while diminishing the listed disadvantages and thereby increase the range of applications for which the engine of the current invention is suitable or at least offer the public a useful choice.
In GT engines the continuous compression and expansion of the gases is typically achieved using one or more compressor and turbine stages respectively. It is these compressor and turbine stages that are the principal source of inefficiency in GT engines.
In each compressor stage the working fluid, normally air, is subjected to an increase in pressure as a result of a work input that generally produces a velocity increase followed by a diffusion process to convert the increased fluid velocity into increased pressure. This process is performed continuously in an axial flow or centrifugal compressor (turbo-compressors) with components rotating relative each other imparting successive velocity and pressure changes to the working fluid.
GT compressors typically have a relatively small range of flow rates over which they can sustain close to their maximum pressure ratio owing to blade stall and sonic choking. This acts to limit the range of power outputs over which high GT engine efficiencies can be maintained. It is therefore an objective of the current invention to offer a method for achieving a continuous compression process in which high pressure ratios may be maintained over a wider range of flow rates with very high efficiencies in both small and large engines, and in which a large range of power outputs from an engine may be efficiently attained, or to at least offer the public a useful choice.
In each GT turbine stage the working fluid, most normally air combined with fuel combustion products, undergoes reductions in pressure and temperature as the expanding gas produces work output. This process can be performed continuously in an axial flow or radial inflow turbines with components rotating relative each other imparting successive velocity and pressure changes to the working fluid.
To achieve high efficiencies some GT engines with over 10 MW output operate with turbine inlet temperatures of up to 1600° C., far beyond the melting temperatures of the materials that they are made of, and relying upon very complex and intricate cooling methods to allow them to survive. Owing to a number of economic and physical factors these cooling methods are not normally applied in smaller engines, thereby typically limiting the maximum combustion temperatures in relatively small gas turbine engines (less than 1 MW output) to less than 1000° C. and consequently limiting achievable efficiencies. It is therefore an objective of the current invention to offer a method for continuous expansion of combustion gases that allows high combustion and gas temperatures to be utilized in relatively low power output engines or at least offer the public a useful choice.
Conventional GT compressors and turbines typically have gases flowing through them at velocities of hundreds of meters per second, and are subject to many inefficiencies and loss mechanisms that reduce the overall compression and expansion efficiencies possible within conventional turbo-machinery. These losses include: mechanical friction in seals and bearings, viscous gas losses that are primarily associated with high flow velocities and/or small cross section flow passages fluid friction due to viscosity of gases, turbulence that dissipates pressure, gas leakage from high to low pressure regions such as blade-tip losses where gases leak over the end of blades from the high to the low pressure side and diffusion losses where a reduction in fluid velocity is imperfectly converted to an increase in pressure and unwanted heat transfer that either adds heat during compression or extracts heat during expansion. Most of these losses are generally greater in smaller compressors and turbines and contribute to the lower efficiency of small gas turbine engines relative to internal combustion engines of similar power output. It is therefore an objective of the current invention to offer an engine in which the overall fluid compression and expansion processes can be achieved with greater efficiency than in conventional compressors and turbines, thereby producing an engine with greater efficiency than a conventional GT engine of similar power output, or to at least offer the public a useful choice.
Rotor and stator blades in turbo-compressor and turbine stages are typically highly loaded and may be required to be formed to precise aerodynamic shapes, in many cases requiring the use of expensive materials and manufacturing processes. In addition, to achieve high engine pressure ratios requires many stages that further increase costs. This may limit the economic viability of GT engines in some applications. It is therefore an objective of the current invention to reduce the overall cost of the components used to achieve continuous compression and expansion processes with high pressure ratios and thereby reduce the overall cost below that of an equivalent conventional GT engine or to at least offer the public a useful choice.
High combustion temperatures, typically above 1600K, can contribute significantly to the formation of undesirable Nitrous Oxides (NOx) emissions, particularly when combined with high pressures and long combustion residence times, the duration for which air is maintained at high temperature during combustion. However, high temperatures and pressures are typically also associated with high engine efficiencies. It is therefore an objective of the current invention to offer the potential for achieving high efficiencies while also limiting the production of NOx, or to at least offer the public a useful choice.
GT engines typically have a very large amount of rotational kinetic energy in the fast spinning compressor and turbine, and the energy required to change the speed of these heavy compressors and turbines typically leads to relatively slow response to changing power demand that may be problematic for some applications.
Large GT engines are frequently combined with steam turbines driven by steam heated by the GT exhaust into Combined Cycle power plants with thermal efficiencies of up to 60%, which discounting fuel cells is the highest efficiency currently widely available for power production from hydrocarbon fuels. They are commonly used for electricity production and in some cases ship propulsion. But while the high efficiency and relatively low cost of Combined Cycle power plants makes them economically attractive for power production, they have some very unattractive attributes as well: In the absence of large volumes of cooling water from a source in the environment cooling towers that utilise the atmosphere for cooling are generally large and impose significant maintenance costs; The amount of stored kinetic energy and stored thermal energy in the steam turbine, steam and other components is very high leading to extremely slow response to changing power demand and start up times that are frequently measured in hours, which may create problems in responding to variable demand or production requirements in an electrical power supply grid and may also occasionally be problematic for ship propulsion. It is therefore an objective of the current invention to offer an engine that has a combination of high efficiency and fast response to power demand changes, or to at least offer the public a useful choice.
Combined Cycle power plants are most efficient in sizes greater than 100 MW output, and lower efficiency in smaller sizes generally leads to bigger sizes being preferred. But such large plants are typically considered unsightly and take up a lot of land area, requiring powerful electrical grid connections, high capacity fuel supplies, and imposing very large localized heat loads on the environment. All of which factors typically lead to the plants being sited far away from cities and towns and taking years to plan, approve and build, while also having increased transmission losses and spending on high-power transmission infrastructure that may together add significantly to the cost of electricity delivered to users. It is therefore an objective of the current invention to provide an engine that can be used to generate electricity at efficiencies competitive with Combined Cycle power plants in far smaller sizes that may be more readily situated close to or within cities and towns, while reducing the time required to install such an engine to far below that required for a Combined Cycle power plant, or to at least offer the public a useful choice.
A wide variety of industrial processes require a supply of compressed air. Compressed air is commonly created by relatively inefficient and expensive processes such as electric motor or engine driven compressors. Additionally electricity typically costs several times as much as fuel for the same amount of energy. It is an therefore an objective of the current invention to provide a means for producing compressed air using a combustible fuel as an energy source and to do so at potentially lower costs than competing air compression technologies, or to at least offer the public a useful choice.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing the preferred embodiment of the invention without placing limitations thereon.
The background discussion (including any potential prior art) is not to be taken as an admission of the common general knowledge.